A secondary lithium ion battery generally contains one or more electrochemical battery cells. Each cell typically includes a negative electrode, a positive electrode, and an intervening membrane that permits an electric current to be reversibly passed between the negative and positive electrodes through an external circuit. To function in this manner, the membrane is designed to physically separate and electrically insulate the confronting faces of the two electrodes—which prevents a short-circuit in the cell—and to exhibit lithium ion conductivity so that lithium ions can migrate between the electrodes and electrochemically balance the external electric current. The membrane may be rendered ionically (e.g., lithium ion) conductive by the presence of an electrolyte. And several different kinds of membranes that can provide a suitable ionic conductivity have been developed.
One type of lithium ion conductive membrane that may be employed is a gel electrolyte. A gel electrolyte generally comprises a polymer host and a liquid electrolyte conducive to lithium ion mobility absorbed (or plasticized) into the polymer host. This type of membrane is different from other types of membranes such as, for instance, a solid polymer electrolyte that includes an ionically-conducting salt integrated into a relatively high-molecular weight polymer, and a porous polymer separator soaked with (but not plasticized to form a gelatinous structure) a liquid electrolyte. Many of the gel electrolytes that are commonly used today include a homopolymer host plasticized with a compatible liquid electrolyte that includes a solvent and a lithium-based salt. The specific homopolymer usually employed is one of poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), or poly(vinylidene fluoride) (PVdF).
A gel electrolyte may be used in an electrochemical battery cell of a lithium ion battery for a variety of reasons. A few specific reasons include the ability to conduct lithium ions without the use of a free liquid electrolyte, the relatively simple packaging requirements, and the flexibility to be fashioned into many different electrochemical battery cell configurations, among others. But the conventional homopolymer-based gel electrolytes used today also have some limitations that need to be considered. In particular, they can suffer thermodynamic and mechanical property declines in response to modest upward fluctuations in cell temperature. A possible explanation for this behavior is the viscosity of a homopolymer-based gel electrolyte may decrease with rising temperatures and, additionally, the solvent used to make the liquid electrolyte may begin to dissolve the homopolymer host. These and other performance-related issues have made the development of a more operationally robust gel electrolyte the subject of ongoing research.